Process for the preparation of high octane gasoline fractions

ABSTRACT

OCTANE MONOCYCLIC AROMATIC PETROLEUM FRACTION SUTIABLE FOR GASOLINE BLENDING.   HIGH OCTANE GASOLINE FRACTIONS ARE PRODUCED IN A PROCESS COMPRISING SEGREGATED CRACKING OF VIRGIN AND RECYCLE STOCKS COUPLED WITH DISTILLATION AND EXTRACTIVE DISTILLATION TO RECOVER ALKYLATION FEESTOCK COMPONENTS AND A HIGH

nited States Patent U.S. Cl. 208-96 10 Claims ABSTRACT 0F THE DISCLOSUREHigh octane gasoline fractions are produced in a process comprisingsegregated cracking of virgin and recycle stocks coupled withdistillation and extractive distillation to recover alkylation feedstockcomponents and a high octane monocyclic aromatic petroleum fractionsuitable for gasoline blending.

Since it may be necessary to reduce or eliminate the use of leadcompositions in fuels, refiners are considering means of providing highoctane fuel components to maintain octane levels adequate for existinghigh compression automotive engines. If lead is eliminated the octaneloss would be about 6-10 numbers and additional processing of refinerygasoline components will be desired to maintain the octane number of thefuels at the required levels. The refinery processing units that arecurrently available to raise octanes are alkylation, reforming,isomerization, catalytic cracking and hydrocracking. Alkylation provideshigh octane parafnic hydrocarbon components and reforming provides highoctane aromatic components.

Since cat cracking is the major processing tool employed in modernrefineries to reduce molecular weight, it would be desirable to use catcracking as the basic step in providing the maximum quantity ofmaterials that can be used directly in gasoline and/ or upgraded byfurther inexpensive treating steps to provide high octane gasolinecomponents.

We have developed a processing technique in which segregated catalyticcracking steps and extractive distillation are integrated to form highoctane naphtha and large yields of alkylation feedstocks. Brieflystated, our invention comprises the steps of cracking a virgin petroleumfeedstock in a transferline type cracking zone, cracking cycle stock ina riser-dense bed cracking zone, distilling the combined crackedeffluents, recovering alkylation feedstock and employing extractivedistillation to recover a high octane monocyclic aromatic hydrocarbonfraction which is suitable for use in motor fuel.

In the process of this invention, fresh feed boiling in the gas oilrange is mildly cracked under conditions which maximize the formation ofC4 olens and which minimize the formation of gas and coke. A segregatedtransferline cracking unit is particularly suited to this type ofcracking operation because it provides high selectivities to olefins.The highly active crystalline molecular sieve catalysts give maximumconversion of the feed in the transferline and they are preferred forthis cracking step. Cracked effluent is separated into a C4-containingfraction that can be further processed to obtain feed for an isoparafnalkylation unit. The next higher boiling fraction which has a nominalboiling range of from about 100 F. to about 425 F. contains gasolineboiling range paraffins, olens, naphthenes and monocyclic aromatics. Themonocyclic aromatic hydrocarbons boiling in the gasoline boiling rangehave side chains containing 1 6 carbon atoms attached to the benzenering. These are desirable high octane components for gasoline. Thesearomatics are separated by extractive distillation and in a preferredembodiment the extractive distillation `solvent is produced ice in situby recovering it from the cat fractionator. The fraction boiling in therange of from about 650 F. to 700 F. comprises three ring aromatichydrocarbons and this material Will selectively separate thedesiredjnonocyclic aromatic hydrocarbons from the nonaromatic material.The monocyclic aromatics can then be recovered from the higher boilingsolvent fraction by a simple flashing step. The nonaromatic material canbe recovered or part of it can be recovered; but in a preferredembodiment this fraction is recycled for further cracking. If themonocyclic aromatics are not separated from the material that is exposedto further cracking as recycle, the result is further condensation inthe cracking unit to aromatics which boil above the gasoline boilingrange. Furthermore, as the aromatic rings condense, hydrogen atoms Willbe transferred to oleiins to form saturates, reducing the quantity ofdesirable low molecular weight olefin product.

In order to obtain incremental conversion, a recycle stream is cracked.The cycle oil is more diflicult to crack than the fresh feed and in theprocess of the invention a dense bed is used to provide residence time.Thus, a cycle oil containing predominantly multi-ring aromatichydrocarbons having one or more side chains of 1-10 carbon atoms iscracked to selectively remove the side chains, producing largequantities of C3-C5 olefins. In a preferred embodiment the cycle oilcomprises mixed distillate from the cat fractionator; e.g., a fractionboiling in the range of from about 480 F. to about 650 F. mixed with asecond fraction boiling in the range of from about 700 F. to about 800F.

This processing scheme has excellent flexibility. A Wide range ofcracking severities can be achieved utilizing segregated transferlinecracking for fresh feed and segregated dense bed cracking for cycle oil.Variations in the feed compositions, catalyst type, the cat to oilratio, and the cracking temperatures can be made to obtain optimum highoctane gasoline components at all times.

Additional advantages and details of the invention will be apparent fromthe following detailed description. The drawing comprises a diagrammaticiiow sheet illustrating a preferred embodiment of the process.

Referring to the drawing, fresh feed is fed by line 1 to the lowermostpart of transferline reactor 2. The feed is mixed `with regeneratedcatalyst flowing in return line 3. Fresh makeup catalyst is added vialine 4. Suitable fresh cracking feedstocks comprise hydrocarbonfractions boiling in the range of 450 F. to 1100" F., preferably 550 F.to 950 Preferred feedstocks include virgin atmospheric gas oils, virginvacuum gas oils, hydrotreated gas oils, coker gas oils, fractions fromsolvent extraction, deasphalted oils and mixtures thereof. The preferredcatalysts for the present process are the crystalline aluminosilicatezeolite types. In general, the chemical formula of the anhydrouscrystalline zeolites employed in the present invention, expressed interms of moles, may be represented as:

wherein Me is selected from the group consisting of metal cations,hydrogen and ammonia, n is its valence and X is a number in the range of2 to 14, preferably 2.5 to 6.5. The crystalline aluminosilicate zeolitesinclude synthetic crystalline aluminosilicates, naturally occurringcrystalline aluminosilicates, and caustic treated aged clays in which aportion of the clay has been converted to crystalline zeolite. Syntheticmaterials include faujasites and mordenites. Natural materials areerionite, analcite, faujasite, phillipsite, clinopt-ilolite, chabazite,gmelinite, mordenite and mixtures thereof. Montmorillonite and kaolinclays can be treated to obtain crystalline aluminosilicates. All or aportion of the cations of the zeolites such as sodium cations can bereplaced with hydrogen ions, ammonium ions, or metal cations such asrare earths, manganese, cobalt, zinc and other metals of Groups I toVIII of the Periodic Table. Matrix type fluid cracking catalysts inwhich the zeolite crystals are coated with or encapsulated in asiliceous gel are preferred zeolite type catalysts.

The mixture or d-ilute suspension of fluidized catalyst and feed invapor or mixed vapor-liquid phase passes upwardly through transferlinereactor 2 at a velocity in the range of from about 6 to about 50 ft. persecond. The length-to-diameter ratio (L/D) of the reactor ranges fromabout 4 to about 50. The space velocity is in the range of 25 to 125w./hr./w. Because the mixture of regenerated zeolite catalyst and freshzeolite catalyst is very active, the fresh feed `in transferline 2 iscracked in a few seconds; i.e., less than about 30 seconds, and moreprobably 0.5- seconds.

Effluent from the transferline reactor is initially separated in roughcut cyclone 5. Separated catalyst passes down the dipleg into the densebed and cracked eflluent passes up through the disengaging zone tocyclone 6.

Segregated cycle oil, the source of which will be discussed hereinafter,`is fed by line 7 into the bottom of riser 8. The cycle oil is mixedwith regenerated catalyst from regenerator 9. The cycle oil is crackedin part in riser 8 and in part in fluidized dense bed 10. Crackedeffluent passes through cyclone 6 into line 11 and catalyst is returnedto the dense bed via the dipleg. Spent catalyst from the transferlinecracking step and from the dense bed is stripped in stripper 12 andpassed by line 13 to regenerator 9. Regeneration is conventional. Fluegas is recovered by line 14.

Cracking conditions in the transferline cracking zone and in the densebed include temperatures in the range of 850 F. to 1050 F. and pressuresin the range of 5 to 35 p.s.i.g. The cycle oil is subjected to moresevere cracking action because of the effect of the relatively longresidence time; i.e., more than about 30 seconds.

The cracked eflluent from the two cracking zones is passed by line 11 tofractionator 15. An overhead fraction having an end point in the rangeof about 390 F. to 430 F. is taken overhead from the fractionator byline 16. This fraction contains substantial quantities of C4hydrocarbons used in isoparaflin alkylation and substantial quantitiesof monocyclic aromatic hydrocarbons which boil in the gasoline boilingrange. The overhead fraction is separated in distillate drum 17. Analkylation feed fraction having an end point in the range of from about100 F. to about 130 F. passes by line 18 to light ends separation unit19. The light ends unit is operated in the conventional manner toprovide any desired type of separation. In this particular embodiment,for example, a gas fraction including C2 minus hydrocarbons and othergases is recovered by line 20. Propane and n-butane are recovered byline 21. A fraction containing `C3 and C5 olefins; i.e., propylene andpentenes, can be fed by line 22 to the alkylation unit, or alternativelyany part of this fraction can be recovered by line 23. A C4 fractioncontaining butenes and isobutane is passed via line 24 to isoparafllnalkylation unit 25. Alkylation is conventional operation with catalystssuch as H2804 and H/F at temperatures in the range of F. to 100 F. andpressures in the range of 2 to 150 p.s.i.g.

Returning to distillate drum 17, a hydrocarbon fraction containingsubstantial quantities of monocyclic aromatic hydrocarbons and typicallyboiling in the range of from about 115 F. to 410 F. is passed from thedrum by line 26 to the lower section of extractive distillation tower27. The predominantly monocyclic aromatic hydrocarbon fraction iscontacted with a solvent under extractive distillation conditions. Inextractive distillation the separation of different components ofmixtures which have similar vapor pressures is effected by flowing arelatively high boiling solvent, which is selective for one of thecomponents in the feed, down a distillation column as the distillationproceeds. The relatively less soluble component passes overhead, whilethe selective solvent scrubs the soluble component from the vapor. Thesolvent containing the dissolved component is withdrawn from the bottomof the column and the dissolved component and solvent may be separatedin an auxiliary unit. Tower 27 can be operated at temperatures in therange of 250 F. to 500 F. and pressures in the range `of 0 to 25p.s.i.g. Conventional features of extractive distillation such asreboiler elements, reflux systems, bleed streams and pumparounds havenot been sho-wn.

Any suitable solvent for monocyclic aromatic hydrocarbons can be used;however, it is a feature of the invention that the solvent can beobtained from the process itself rather than from an external source.Specifically, the solvent can be obtained by recovering a particularfraction from cat fractionator 15. In a preferred embodiment, thesolvent is an aromatic hydrocarbon fraction containing a major amount ofthree ring aromatic hydrocarbons. Thus, line 28 passes an aromaticfraction boiling in the range of from about 650 F. to about 700 F. tothe upper portion of tower 27. The multicyclic aromatic fraction passesdownwardly through the tower extracting monocyclic aromatics from theextractive distillation feedstock. The extract fraction is passed vialine 29 to flash tower 30. The monocyclic aromatic gasoline fraction isflashed overhead from tower 30 for recovery by line 31. The solvent isrecycled via line 32. The nonaromatic raffinate from tower 27 can berecycled to the dense bed cracking step by line 33 and/or any portion ofit can be recovered by line 34 as a product of the process. Since tworing aromatic hydrocarbons with fewer than three carbon side chains arenot desirable recycle materials, an aromatic fraction of this type isremoved from the process by line 3S. For operations with insufficientcoke make, part or all of this stream may be recycled. The monocyclicaromatic hydrocarbon fraction recovered from the process Will have anunleaded octane number of from about 96 to about 102.

In this embodiment the cycle oil comprises two components fromfractionator 15. A fraction boiling in the range of 700 F. to 800 F. ispassed by line 36 to line 7 for admixture with a fraction boiling in therange of 480 F. to 650 F. and the mixed fractions are recycled to thedense bed reactor. The composition of the cycle oil is optional and anyfraction or mixture of fractions amenable to dense bed cracking can berecycled for this purpose.

One of the major benefits of the process of the invention is that itprovides an efficient, inexpensive means of recovering high octanearomatic motor fuel components as well as large quantities of alkylationfeed. Recracking of desirable aromatic naphtha components issubstantially reduced. Segregated cracking of selected fractions at theconditions that are most suitable for each fraction provides moreefficient conversion to materials that can be used to raise gasolineoctane without depending on lead compositions for octane boost.

What is claimed is:

1. A process for the preparation of a high octane aromatic petroleumfraction comprising the steps of:

(a) cracking a fresh petroleum feedstock in a transferline crackingreactor in the presence of a crystalline zeolite cracking catalyst toprovide a first cracked effluent;

(b) cracking a recycle petroleum fraction in a riserdense bed crackingreactor in the presence of a crystalline zeolite cracking catalyst toprovide a second cracked effluent;

(c) combining said rst cracked effluent and said second cracked effluentto provide a combined eflluent;

(d) passing said combined eluent to a distillation tower;

(e) recovering at least two petroleum fractions from said distillationtower including:

(i) an overhead fraction boiling below about 40G-450 F.; and (ii) arecycle fraction;

(f) separating said overhead fraction into a hydrocarbon fractioncontaining substantial quantities of C4 hydrocarbons and a hydrocarbonfraction containing substantial quantities of monocyclic aromatichydrocarbons;

(g) contacting the monocyclic aromatic hydrocarbon fraction with asolvent for aromatic hydrocarbons at extractive distillation conditions;and

(h) recovering a monocyclic aromatic hydrocarbon fraction from saidsolvent.

2. Process according to claim 1 in which said fresh petroleum feedstockcomprises a gas oil.

3. Process according to claim 1 in which said catalyst comprises acrystalline zeolite encapsulated in a siliceous gel matrix.

4. Process according to claim 1 in which said recycle petroleum fractioncomprises a major amount of multiring aromatic hydrocarbons having sidechains of 1-10 carbon atoms.

5. Process according to claim 1 in which said hydrocarbon fractioncontaining substantial quantities of C4 hydrocarbons is subjected tolight ends treatment to concentrate said C4 hydrocarbons and said C4hydrocarbons are fed to an alkylation unit for alkylation in thepresence of an alkylation catalyst.

6. Process according to claim 1 in which the solvent employed in step(g) comprises an aromatic hydrocarbon fraction boiling in the range offrom about 650 E. to about 700 F. containing predominantly three ringaromatic hydrocarbons.

7. Process according to claim 1 in which said monocyclic aromatichydrocarbon is separated from said solvent by flashing.

8. Process according to claim 1 in which the monocyclic aromatichydrocarbon fraction recovered from step (h) has an unleaded octanenumber of at least 96.

9. A process for the preparation of alkylation feedstock components andan aromatic petroleum fraction boiling in the gasoline boiling rangecomprising the steps of:

(a) cracking a virgin gas oil at mild cracking conditions in a fluidizedtransferline cracking reactor in the presence of a crystalline zeolitecracking catalyst;

(b) cracking a recycle petroleum fraction in a riserdense uidized bedcracking reactor in the presence of a crystalline zeolite crackingcatalyst;

(c) combining the elluent from the two cracking reactors in thedisengaging zone of the reactor of step (b);

(d) distilling the combined effluent to recover a plurality of fractionsincluding:

(i) an overhead fraction boiling below about 400- 450 F.; (ii) anaromatic fraction containing a major amount of two ring aromatics; (iii)at least one recycle fraction; v (iv) a solvent fraction comprising amajor amount of three ring aromatic hydrocarbon compounds;

(e) separating said overhead fraction (d) (i) into an alkylation feedfraction containing a major amount of butenes and isobutane and apredominantly aromatic fraction containing substantial quantities ofmonocyclic aromatic hydrocarbons;

(f) contacting said monocyclic aromatic fraction of step (e) with thesolvent fraction of step (d)(iv) at extractive distillation conditions;

(g) recovering an aromatic extract fraction and a Vaporized nonaromaticrainate fraction;

(h) separating a monocyclic aromatic fraction boiling below about40G-450 F. from said aromatic extract fraction; and

(j) recycling at least a portion of the remaining extract fraction tostep (f) as solvent.

10. Process according to claim 9 in which said recycle fractionrecovered from step (d)(iii) comprises a mixture of a distillatefraction boiling in the range of about 480-650" F. and a distillatefraction boiling in the range of about 700-800 F.

References Cited UNITED STATES PATENTS 2,702,782 2/ 1955 Little 208-962,941,936 6/ 1960 Harper 208-96 3,190,828 6/1965 Daniel et al. 208-783,193,488 7/1965 Carr 208-96 3,303,123 2/1967 Payton et al. 208-963,448,037 6/1969 Bunn et al. 208-164 HERBERT LEVINE, Primary ExaminerU.S. Cl. X.R.

